The present invention relates generally to snubber circuits, and more particularly to a snubber circuit which utilizes an inductively coupled resistance to dampen oscillations in the snubber circuit while minimizing added inductance.
In conventional power inverters, a pair of power switches connected in series between positive and negative DC terminals are utilized to operate during opposite half cycles to provide an AC output. Snubber circuits are often used to reduce the voltage spikes and power losses that occur as a result of turning the power switches on and off. The snubber circuits absorb the voltage spikes, and therefore reduce the power losses and overall switching losses that result from repeatedly switching the power switches.
In many inverter applications, snubber circuits include capacitors which are placed directly across the switching devices in parallel in order to reduce switching losses. One problem with this arrangement is that the capacitor, while greatly reducing the switching losses, creates an oscillatory circuit when placed in parallel to a DC power bus. The result is unwanted "ringing" in the circuit. FIG. 1 shows an example of a prior art snubber circuit 10 in which electrical oscillations or ringing occurs as a result of switching switches S1 and S2. The oscillations occur along the paths indicated by arrows 12, 14 and 16 at a frequency related to stray inductance L1 and the series capacitance combination of C1, C2 and filter capacitor C.sub.f.
It has been found that placing a resistor in series with snubber capacitors C1 and C2 significantly reduces the oscillatory ringing in high frequency switching circuits. However, in the case of high frequency inverter circuits which utilize IGBT switches, the use of such resistors in snubber circuits is limited because of the high power requirements of the inverter circuits. Particularly in industrial applications, such a resistor placed in series with the capacitor must not only have a high power rating and a low inductance, but it must be a relatively low value component. The use of such resistors in high power snubber circuits is commercially impractical because of the lack of readily available low value resistors having the required high power rating.
Moreover, certain packaging and other physical limitations exist with respect to the amount of space between the capacitor and the DC power bus. In some applications, it is not feasible to physically fit such a discrete resistor with the required power rating in the snubber circuits from a manufacturing point of view, given limited space constraints. Additionally, a resistor with such high power ratings dissipates heat such that it is necessary to water cool the resistor. This, in turn, requires an even larger space for any associated cooling apparatus for the resistor.
Further, in a typical inverter, inserting a small value resistor in series with the capacitor introduces a significant amount of inductance into the snubber circuit.
Therefore, it would be desirable to have a snubber circuit that 1) inductively couples a resistor to induce a series resistance with a snubber capacitor to reduce circuit switching losses and oscillations without introducing additional inductance, 2) utilizes high power/high value resistors that are economical and commercially available, and 3) avoids the physical limitations of introducing such a resistor into the wire leads of the snubber capacitor.